The present invention relates to a position detecting system for a piston/cylinder assembly and the method of determining the specific location of a piston rod and cylinder relative to each other.
Piston-cylinder assemblies are used in various actuator applications throughout industry. It is advantageous for an operator to be aware of the specific position of a piston rod in a fluid pressure operated cylinder since the working member being actuated is generally physically connected to the distal end of the piston rod. In order to effectively accomplish this task, several aspects have to be included in the design of such an assembly. Specifically, a readable scale has to be applied to the piston rod, and a sensing device capable of reading the scale has to actually read the scale and communicate this reading, in terms of the specific location of the piston rod, to the operator.
In the past, surfaces have been marked in various ways. Lasers have been used to treat surfaces in order to provide readable scales. An example of a laser marking technique is shown in U.S. Pat. No. 4,547,649 to Butt et al. This patent adds an oxidized layer to a surface and then makes color contrasting marks on this surface that can be detected. Other examples of using lasers to create markings are shown in the following prior art: U.S. Pat. No. 4,533,815 to Ecer; U.S. Pat. No. 4,758,705 to Hertzel et al.; U.S. Pat. No. 4,170,726 to Okuda; U.S. Pat. No. 4,335,295 to Fowler; U.S. Pat. No. 4,347,785 to Chase et al.; U.S. Pat. No. 4,406,939 to Golker; U.S. Pat. No. 5,298,717 to DeRossett, Jr.; U.S. Pat. No. 5,424,508 to Swain et al.; and U.S. Pat. No. 5,886,317 to Hinrichs et al. These prior art patents all use laser technology to remove material from the treated surface. Removal of material from the piston rod surface can create difficulties with providing a complete seal around the piston rod. Without a complete seal, pressure can be lost at these areas and contaminants can infiltrate the system.
In order to ensure that the piston-cylinder assembly is properly sealed, other inventions have treated surfaces without altering the surface geometry of the piston rod. This type of surface treatment is shown in prior art U.S. Pat. No. 5,632,916 to Lappalainen et al. This invention also uses a laser to form a scale without altering the surface geometry that is being marked.
Various other coding scales have been applied to surfaces in the past. For example, U.S. Pat. No. 4,901,073 to Kibrick uses a bar code for determining the position of a movable member relative to a stationary member. U.S. Pat. No. 4,701,615 to Schmitt details the use of a binary code that assists in determining the direction of travel and the position of two objects relative to each other. U.S. Pat. No. 5,632,916 to Lappalainen et al., previously referenced, shows a method of using a laser to create discolorations on a metal surface that can be optically read. In these cases, the treated surfaces are not geometrically altered.
As previously mentioned, a proper seal is required to ensure that the pressure inside the piston-cylinder assembly is not lost. A seal also can provide a way of eliminating any outside contaminants that may possibly enter the enclosed piston-cylinder assembly via reciprocation between the piston rod and the cylinder. The prior art has identified various ways of providing a proper seal. The following prior art sets forth sealing techniques for cylindrical objects: U.S. Pat. No. 3,013,826 to Sharp; U.S. Pat. No. 3,284,088 to Pippert; U.S. Pat. No. 4,055,107 to Bartley; and U.S. Pat. No. 5,607,165 to Bredemeyer.
In order to determine the specific position of the piston rod, a sensor is used to read the coding scales. Different types of sensors have been used in the past. For example, U.S. Pat. No. 5,172,485 to Gerhard et al. uses a position sensor that measures the capacitance between two objects. In U.S. Pat. No. 5,455,509 to Semura et al., a magnetic sensor is used to detect the stroke position of a cylinder. U.S. Pat. No. 6,234,061 B1 to Glasson uses a rotating spool to communicate with a transducer in order to sense the position of a piston rod. U.S. Pat. No. 5,539,993 to Kilpinen sets forth a light emitting and light receiving fiber optic sensor to detect the absolute position of a moving object. Finally, a report titled xe2x80x9cPosition transducers provide system feedbackxe2x80x9d in the industry magazine Hydraulic and Pneumatics, April 2000 edition, starting on page 39, sets forth several devices that provide position information for fluid-power control systems.
The present invention provides a compact, accurate position detecting system for a piston/cylinder assembly. This invention utilizes essentially existing piston-cylinder componentry and adds the capability of determining the exact location of a piston rod relative to its surrounding cylinder. This invention overcomes the prior art""s inability to provide a simple, compact apparatus and method for detecting the position of a piston rod and cylinder relative to each other.
A feature of the present invention is to incorporate a sensor within the existing componentry. This invention overcomes difficulties in the prior art, which required the addition of an external sensor to the piston-cylinder assembly. External sensors are subject to harmful outside elements that can adversely affect the sensor""s accuracy and shorten the sensor""s life. For example, piston-cylinder assemblies are typically used on construction equipment. External componentry can come in contact with contaminants and into physical contact with various external objects, thus causing damage. Since a sensor is a precise instrument, any damage will impair or even totally eliminate its functionality. External componentry also adds unwanted size and complexity to the piston-cylinder assembly. In the present invention, the sensor is housed within an existing component, namely a sealing gland, which encapsulates the open end of the cylinder and receives a moving piston rod. By incorporating the sensor within the sealing gland, the size of the piston-cylinder assembly is generally not increased and the sensor is isolated from harmful elements.
Another feature of the present invention is to separate the sensor from external contaminants and internal pressure within the cylinder body. This is accomplished by providing seals within the seal gland, on both sides of a cavity that houses the sensor, thus isolating the sensor from the internal cylinder pressure and any external contaminants that may accumulate on the piston rod during its extension from the sealing gland.
Another feature of the present invention is to provide an easy way of removing and inserting the sensor. Due to its location within the seal gland, the piston-cylinder assembly does not have to be disassembled in order to replace the sensor. In addition, during the replacement of the sensor from the seal gland, the piston/cylinder assembly will not lose pressure.
Another feature of the present invention is to determine the position of the piston rod relative to the cylinder without having to add an additional, externally mounted scale. This is preferably accomplished by subjecting the piston rod surface to a change in its molecular structure, resulting in the addition of a readable code to its surface. This code, for example in the form of indicia markings, is read by the sensor and can determine the position of the rod and the cylinder relative to each other. This is done without having to add another component.
A further feature includes adding the readable code, for example in the form of an incremental scale, to the piston rod surface without changing its surface geometry. This ensures a proper seal surrounding the piston rod. The prior art has provided codes or indicia markings that are laser etched into the surface of the piston rod. Laser etching alters the surface geometry of the piston rod, thus provided a leak path, around the seals, for fluids and contaminants. Laser etching of the surface geometry can also damage the seals and thus provide a leak path for the fluid and/or contaminants.
Another feature, as shown in a further embodiment, is that the piston rod surface can be treated at specific locations in order to provide an xe2x80x9cend-of-strokexe2x80x9d, and/or xe2x80x9cmid-strokexe2x80x9d, types of sensing. Certain applications may not need to determine the absolute position of the piston rod at all times during its stroke. Therefore it is a cost benefit to only include indicia markings at specific locations on the piston rod. For example, an operator of a piston/cylinder assembly may only want to know when the piston is in mid-stroke and/or at full stroke; therefore indicia marks can be included on the piston rod surface for defining the noted locations.
Another feature, as is shown in yet another embodiment, is that the position of the piston rod can be instantaneously determined. This is accomplished via an absolute scale encoded on the piston rod surface. In this embodiment the sensor is also positioned inside the sealing gland so that an external sensor component is not needed.
This invention also provides a method of determining the specific location of the piston rod and cylinder relative to each other. This is accomplished by providing markings on the piston rod; mounting a sensor within the cylinder seal gland; moving one of the piston and cylinder relative to the other; reading the markings via the sensor during the noted movement; and conveying the readings to an operator.
As previously described, the features of the present invention provide a compact, accurate position detecting system without having to add any external componentry to the piston-cylinder assembly. In this way, the size of the assembly is generally not increased and the sensor is isolated from any harmful elements, both internal and external. Further features and advantages of the present invention will become apparent to those skilled in the art upon review of the following specification in connection with the accompanying drawings.